My metric right now is page-turn ratio: how much of my reading time is spent waiting for page turns. With paper books is under 10%, with a Sony 505 it was over 20%. That is enough to be annoying, and make me want something with a bigger screen and faster page turns. So I'm not buying a replacement for my 505 until I can get something significantly better. Even A4 (two A5 pages) with the current one second page turn would be ok, but surely it's going to be faster than that.